FIG. 7 shows a known example of the above type of driving circuit for IGBT. In FIG. 7, the driving circuit includes a power supply 1 for producing forward bias current, power supply 2 for producing reverse bias current, MOS field-effect transistor (MOSFET) 3 for forward biasing IGBT, MOSFET 4 for reverse biasing IGBT, gate resistor 5 of the forward-bias MOSFET, gate resistor 6 of the reverse-bias MOSFET, gate resistor 7 for limiting forward bias current, gate resistor 8 for limiting reverse bias current, IGBT 9, gate/emitter resistor 10 of the IGBT 9, and a drive signal input portion 11.
To turn on the IGBT 9 in the circuit as described above, an ON signal is input to the drive signal input portion 11, thereby to turn on the MOSFET 3, and a voltage is applied from the forward bias power supply 1 to the IGBT 9, through the MOSFET 3 and resistors 7, 10, so as to charge a gate capacitor (not illustrated) of the IGBT 9.
To turn off the IGBT 9, on the other hand, an OFF signal is input to the drive signal input portion 11, thereby to turn on the MOSFET 4, so that the gate capacitor of the IGBT 9 is discharged, through the MOSFET 4, resistor 8 and others.
The known driving circuit shown in FIG. 7 does not have a function of changing dv/dt of the IGBT from the outside. To change the rate of change of voltage dv/dt , therefore, it is necessary to individually control or adjust resistance values of the forward bias resistor 7 and reverse bias resistor 8 shown in FIG. 7.
It has been known in the art that the rate of change of voltage dv/dt can be changed by changing the rate or speed of charging and discharging the gate capacitor of the IGBT. Since the capacitance of this gate capacitor at the time of turn-on is different from that at the time of turn-off, the resistance values of the forward-bias resistor 7 and reverse-bias resistor 8 need to be individually adjusted in the circuit of FIG. 7 so as to deal with the difference. However, the adjustment of the resistance values requires a time-consuming, cumbersome procedure, and thus dv/dt cannot be easily changed. This also makes it difficult to reduce high-frequency leakage current.